State of the Art
The treatment of water to render it safe for drinking has become complex process, due to the variety and complexity of water pollutants, and different processes required to remove them. The pollutants of main concern are (1) heavy metals, such as lead and copper, (2) chlorine, disinfection byproducts, and other organic pollutants, and (3) microorganisms.
Heavy Metals
The elevated concentration of lead in drinking water has become a major public health concern. A recent sampling by the U.S. Environment Protection Agency (EPA) of 660 large public water systems found that about 32 million Americans in 130 cities drink water from systems that exceed the federal action limit of 15 parts per billion and that 10 cities exceed the limit by as much as 5 times. The natural waters in many small towns and rural areas in the United States, especially in the southwest, often contain a high concentration of lead and copper.
The long-term intake of excessive lead in water, along with exposure from lead-based paint and contaminated soil and dust, can build up in blood to result in a concentration of this toxic metal to a harmful level. Lead is known to severely hamper physical and mental development in children, to raise blood pressure and interfere with hearing and, at a very high level, to cause kidney damage and mental retardation in adults. In reaction to these findings, the EPA's rule concerning lead in drinking water has become much more stringent, requiring some 79,000 public water supply systems to monitor lead levels at the tap and setting an action level of 15 ppb. The lead may be present in the water as ions, solid particles or be in the water as a soluble complex.
There are various methods to separate lead from aqueous solutions. The separation processes employed usually involve (1) ion exchange, (2) adsorption, (3) reverse osmosis, and (4) coagulation and precipitation.
Ion exchange is a process by which a given ion on an exchange solid is replaced by another ion in the solution, and is often used in processes for control of soluble metals, such as lead. For example, it is known that an ion exchange resin in calcium form can reduce lead in household drinking water, but the resin often lacks the ability to remove lead to the very low level required. Solid minerals can also be used as an ion exchange medium. For example, Takeuchi et al. ("A Study Equilibrium and Mass Transfer . . . ," Journal of Chemical Engineering of JAPAN, 21:1 pp. 98-100, 1988) discloses batch adsorption experiments using solid hydroxylapatite (Ca.sub.5 (PO.sub.4).sub.3 OH). Heavy metals, including lead, were removed from distilled water spiked with the metals by an equilibrium mass transfer of metals between the solid and aqueous phase. The removal of heavy metals was attributed to the ion exchange process in which the surface calcium was replaced by a divalent metal, e.g., Pb.sup.2+.
Adsorption processes usually exploit the Weak van der Waals forces which are responsible for many reversible adsorptions of solutes to solid surfaces, or may involve more specific processes such as ion exchange and/or surface complexation. The weak physical adsorption can be easily reversed upon changes in conditions such as concentration of the solute, pH, temperature, or saturation of surface sites. Prior-art adsorption processes include using a granular activated carbon fixed-bed in a canister as a point-of-use device to remove lead from drinking water. Lead from a solution has also been adsorbed upon the surface of Vermiculite (a mica), Montmorillonite (a bentonite clay) and Goethite (an iron oxide).
Reverse osmosis has been used in point-of-use devices for removing lead from drinking water, as disclosed in Consumer Reports ("Water Treatment Devices," February 1993, pp. 79-82). Also disclosed are devices using distillation, and filtration.
Precipitation, where selected chemicals are applied to cause the solubility of solids to be exceeded, has been used to separate lead from the aqueous phase. Most particularly, carbonate or hydroxide precipitation has been proposed to remove heavy metals from solution. For example, calcium carbonate added to lead solutions has been used to remove lead as a precipitate. It has also been proposed to remove lead by coagulation and flocculation with alum at pH 8 to 9.
In U.S. Pat. No. 5,098,579 to Leigh et al. a method is proposed for continuously treating water by contacting the water with a metal salt which is sparingly soluble in water and has a very strong affinity to react with the ions to be removed to form an insoluble salt. The choice of the sparingly soluble salt is based upon the properties of the ion to be removed. For removal of Pb.sup.2+ ions the sparingly soluble salt may be any of various carbonate salts, chromate salts, Ca.sub.3 (PO.sub.4).sub.2, CaSO.sub.4, or mixtures thereof.
The precipitation processes, such as Leigh et al., are directed mainly to industrial waste streams, and the like. Typically, prior-art precipitation treatments of water were not designed to remove lead to an extremely low value, such as to 15 ppb, and were not designed to function in a point-of-use home culinary system. An ideal point-of-use device for removing lead from drinking water should be capable or removing lead to a concentration of 15 ppb or lower. In addition, it should be relatively inexpensive, mechanically simple and not involve much maintenance. A device that is awkward to apply to existing drinking water systems in the household has a short service life or requires frequent recharging, involves handling of chemicals, particularly hazardous solids and liquids, or is expensive to purchase or maintain is not suitable. Such a device will likely not be used at all and will be eventually discarded or misused by the consumer. In addition, a point-of-use system should provide some indication when its lead removing ability is exhausted. For example, in systems using adsorbents, the like, there is usually no indication when the adsorbent is approaching saturation and becoming ineffective in removing lead sufficiently from the drinking water.
Copper is also known to be toxic at relatively low concentrations in drinking water. Copper in drinking water can occur particularly in areas where ground water supplies have been polluted by mining and smelting activity, and by corrosion in drinking water systems, such as in household piping, faucet and fixtures. Even though copper is a nutritional element, a high concentration of copper can cause gastrointestinal effects. Long term intake of a high concentration of copper in drinking water leads to intestinal, stomach distress and Wilson's disease. A high concentration of copper can also result in acute copper poisoning. Maessen, et al., AWWA, June 1985, 6, 73-80 discloses disorders caused by copper exposure, and it is reported that the death of a 14-month-old child was suspected as being caused by chronic copper poisoning. In reaction to these findings that long term exposure of excessive copper causes a variety of physiological and psychological disorders, the EPA has set an action level for copper of 1.3 ppm. As with lead, copper may also be resent in the water as soluble ions, copper-containing particle, or as a soluble complex.
Disinfection Byproducts
Chlorine is one of the most commonly used disinfectants for the destruction of harmful and pathogenic organisms that might endanger human health. Many of the organic compounds in water may react with the chlorine to form toxic compounds that cause adverse effects to humans after long-term intake. The most commonly seen disinfection byproducts (DBPs) in drinking water are trihalomethane (THM), trichloromethane (TCM), haloacetic acid, and dichloroacetate (DCA). Concentrations of TCM and DCA in drinking water have been reported to be as high as 600 ppb and 100 ppb, respectively. Human exposure to DBPs is rarely acute, but there are several chronic effects cause by exposure over many years. Some of the DBPs have been identified as carcinogens or potential carcinogens. Therefore, cancer has become a major concern to the general public because of chronic exposure to these compounds. The National Cancer Institute has identified TCM as a carcinogen, and researchers have reported DCA and other DBPs as carcinogens or potential carcinogens. DCA is potentially a potent inducer of hepatic tumors and liver cancer. Long-term intake of excessive trihalomethane has been shown to be potentially mutagenic and cytotoxic. As a result, the water treatment and supply industry is facing technical and fiscal hurdles as requirements are made to remove these disinfection byproducts from drinking water, and the maximum allowable concentrations are reduced. Recently, the EPA in the United States has approved a rule to reduce the maximum allowable THM concentration in treated water from 100 to 80 ppb.
Microorganisms
Drinking water, to be safe, should be free of microorganisms that cause health problems, which are most commonly Giardia and Cryptosporidium. Giardia is a protozoan parasite that infects the small intestine of man and many different species of mammals. The parasite is often found among children in kindergartens, nurseries, day care centers, and the like. Transmission is usually though water, so Giardia has become a significant waterborne disease. Since 1980, several outbreaks have occurred, many due to ineffective filtration of the water supply. While not conclusive, data from filtration plants indicates that removal of 75 to 80 percent of the incoming turbidity is necessary to insure removal of the giardia cysts (McFeters, Drinking water Microbiology, 1990, p.279).
Cryptosporidium is a protozoan parasite and can be found in drinking water. Some clinical results show that cryptosporidium will cause profuse watery diarrhea and fluid losses averaging three liters per day. Abdominal pain, nausea, vomiting, and fever may also be present. The symptoms will show on an average of three to six day after exposure. The common mode of transmission is through infected drinking water. Cryptosporidium occurs in water as a oocyst averaging 4 to 5 .mu.m in diameter, much smaller than Giardia.
Because of their flexibility, harmful microorganisms can pass through a filter that is smaller than their diameter, but a filter of 1 .mu.m or less, will filter out essentially all of the giardia and cryptosporidium present in the water. However, to be useful, the filter cannot become quickly clogged from other components in the water, such as precipitates, and the like.